FIG. 1 is a schematic diagram of a conventional commercial nuclear power reactor and various safety and cooling systems for the same. As shown in FIG. 1, a reactor 10 is positioned inside of a containment structure 1. During operation of reactor 10, liquid water coolant and moderator enters the reactor 10 through main feedwater lines 60 that are typically connected to a heat sink and source of fluid coolant, like a condenser cooled by a lake or river. Recirculation pump 20 and main recirculation loops 25 force flow of the liquid down through a bottom of the reactor 10, where the liquid then travels up through core 15 including nuclear fuel. As heat is transferred from fuel in core 15 to the liquid water coolant, the coolant may boil, producing steam that is driven to the top of reactor 10 and exits though a main steam line 50. Main steam line 50 connects to a turbine and paired generator to produce electricity from the energy in the steam. Once energy has been extracted from the steam, the steam is typically condensed and returned to the reactor 10 via feedwater line 60.
In the instance that recirculation pump 20 fails and/or liquid coolant from main feedwater lines 60 are lost, such as in a station blackout event where access to the electrical grid is cut off, reactor 10 is typically tripped so as to stop producing heat through fission. However, significant amounts of decay heat are still generated in core 15 following such a trip, and additional fluid coolant may be required to maintain safe core temperatures and avoid reactor 10 overheat or damage. In these scenarios, active emergency cooling systems, such as a Reactor Core Isolation Cooling (RCIC) turbine 40 or higher-output High Pressure Injection Cooling (HPIC) turbine, for example, operate using steam produced in core 15 by decay heat to drive turbines. Flow from main steam lines 50 is diverted to RCIC lines 55 in this instance. RCIC turbine 40 may then drive an RCIC pump 41, which injects liquid coolant from a suppression pool 30 or condensate storage tank 31 into main feedwater line 60 via RCIC suction line 35 and injection line 42. The injected liquid coolant maintains a coolant level in reactor 10 above core 15 and transfers decay heat away from core 15, preventing fuel damage. Saturated steam coming off RCIC turbine 40 can be condensed in suppression pool 30 by venting into suppression pool 30 via RCIC exhaust line 43.
RCIC turbine 40 typically requires a minimum steam pressure of 150 pounds/square inch in order to drive RCIC pump 41 to inject liquid coolant into main feedwater line 60 via injection line 42 and suction line 35. Pressure in main steam lines 50 from an outlet of reactor 10 will typically drop below 150 pounds/square inch after 8-20 hours of shutdown, at which time RCIC turbine 40 and other higher-pressure injection systems will not function. At this time, lower-pressure shutdown coolant injection systems (not shown) are activated and run off electricity from the electrical grid, or, in the station blackout event, emergency diesel generators. As long as an electricity source is available, lower-pressure injection systems can maintain safe temperatures and fluid level in core 15 until cold shutdown can be achieved or transient circumstances have ended and core 15 can resume generating power through fission. Regulatory bodies worldwide typically require these active systems, including RCIC systems and electricity-powered lower-pressure delivery systems, as the sole mechanisms to avoid core overheat and damage in transient scenarios involving loss of coolant and/or loss of offsite power.